The present invention relates to a process for simultaneously reducing the amounts of hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gas from an internal combustion engine by passing the exhaust gas over a catalyst formed of a gas-permeable inert support with a catalytically active coating which contains at least one high surface area support material, at least one metal from the platinum group, and optionally one or more base metal compounds.
In order to simultaneously remove pollutants in the exhaust gas from internal combustion engines, hydrocarbons and carbon monoxide have to be oxidized in an oxidation reaction and at the same time the nitrogen oxides have to be reduced in a reduction reaction. These conflicting requirements are only satisfied by using so-called three-way catalytic converters with simultaneous control of the standardized air/fuel ratio, represented by lambda, to a value of approximately 1. The standardized air/fuel ratio lambda is defined as the ratio of the actual amount of air required for 1 kg of fuel to the amount of air for stoichiometric combustion of the fuel. Only with lambda values of approximately 1 can degrees of conversion of approximately or greater than 90% be produced for all three types of pollutants (i.e., hydrocarbons, carbon monoxide, and nitrogen oxides) using three-way catalytic converters.
For modern engines which use petrol and/or gas (e.g., so-called lean-burn engines), there are engine production designs which operate with standardized air/fuel ratios greater than 1 (i.e., with an excess of air and thus also of oxygen) with the objective of saving fuel. This excess oxygen is also found in the exhaust gas.
The exhaust gas from lean-burn engines has a proportion of oxygen in the exhaust gas of more than 1 vol.% during the greater part of the operational life of the engine under normal driving conditions. This so-called "lean" operating mode may change to the "fat" or stoichiometric mode with an excess of fuel during relatively short acceleration phases. The mode of operation of lean-burn engines therefore resembles that of diesel engines, whose exhaust gas also normally contains more than 1 vol.% of oxygen.
Known exhaust gas purification processes, under lean-burn exhaust gas conditions, enable only the hydrocarbons and carbon monoxide contained in the exhaust gas to be converted into water and carbon dioxide by oxidation with the assistance of an oxidation catalyst. Thus, there is the risk that nitrogen monoxide (NO) also contained in the exhaust gas is partially converted by oxidation into nitrogen dioxide (NO.sub.2) which has negative effects of the environment. Furthermore, there is the risk that sulphur dioxide (SO.sub.2) also present in the exhaust gas is further oxidized to sulphur trioxide (SO.sub.3) which has negative effects of the environment, too. Special catalyst formulations have been developed to avoid these unwanted oxidations.
Thus, for example, DE 39 40 758 (U.S. Pat. No. 5,157,007 which is incorporated by reference in its entirety) describes a catalyst for the oxidative purification of exhaust gases from diesel engines with a high capacity for converting hydrocarbons and carbon monoxide at low temperatures and with an inhibited oxidizing effect towards nitrogen monoxide and sulphur dioxide. This catalyst contains a catalytically active coating, which consists of finely divided, high surface area aluminum oxide, titanium oxide and/or silicon dioxide on the channels through which the gas freely flows in a solid, honeycomb-shaped, inert support made of ceramic or metal.
A high surface area support material in the context of the present invention and DE 39 40 758 is understood to be a material whose specific surface area (determined in accordance with DIN (German Industrial Standard) 66132 by Brunauer, Emmet and Teller's method (BET surface area)) is larger than 10 m.sup.2 /g.
These finely divided, high surface area substances must be thermally stable in order to prevent the specific surface area of the materials from being essentially reduced at the operating temperatures of the catalyst and thus decreasing the catalytic activity of the catalyst. Substances which are sufficiently thermally stable are those which still qualify as having high surface areas even after several hours of thermal stress at temperatures of at least 700.degree. C.
Onto the finely divided, high surface area substances from DE 39 40 758 are deposited platinum and/or palladium and a vanadium component as catalytically active components. The vanadium component in this catalyst largely prevents the further oxidation of nitrogen monoxide and sulphur dioxide. With this catalyst, hydrocarbons and carbon monoxide are highly and efficiently converted into harmless components. However, reduction of nitrogen oxides to nitrogen does not occur, and nitrogen monoxide and sulphur dioxide pass through the catalyst virtually unchanged.